Stability and structure of Li(n)H molecules (n=3-6): experimental and density functional study

The molecules Li(3)H and Li(4)H have been identified in mass-spectrometric measurements over solutions of hydrogen in liquid Li, and the gaseous equilibria of the reactions: Li(3)H+Li=Li(2)H+Li(2), Li(3)H+Li(2)=Li(2)H+Li(3), Li(3)H+Li=LiH+Li(3), Li(3)H+LiH=2Li(2)H, and Li(4)H+Li(2)=Li(3)H+Li(3) have been measured. Density functional calculations of Li(n)H molecules (n=3-6) provide structures, vibrational frequencies, ionization energies, and free energy functions of these molecules, and these are used to estimate the enthalpies of these reactions and the atomization energies of Li(3)H (119.4 kcal/mol) and Li(4)H (151.8 kcal/mol).     

Molecular structures and energetics of the (TiO2)n (n = 1-4) clusters and their anions

The (TiO2)n clusters and their anions for n = 1-4 have been studied with coupled cluster theory [CCSD(T)] and density functional theory (DFT). For n > 1, numerous conformations are located for both the neutral and anionic clusters, and their relative energies are calculated at both the DFT and CCSD(T) levels. The CCSD(T) energies are extrapolated to the complete basis set limit for the monomer and dimer and calculated up to the triple-zeta level for the trimer and tetramer. The adiabatic and vertical electron detachment energies of the anionic clusters to the ground and first excited states of the neutral clusters are calculated at both levels and compared with the experimental results. The comparison allows for the definitive assignment of the ground-state structures of the anionic clusters. Anions of the dimer and tetramer are found to have very closely lying conformations within 2 kcal/mol at the CCSD(T) level, whereas that of the trimer does not. In addition, accurate clustering energies and heats of formation are calculated for the neutral clusters and compared with the available experimental data. Estimates of the titanium-oxygen bond energies show that they are stronger than the group VIB transition metal-oxygen bonds except for tungsten. The atomization energies of these clusters display much stronger basis set dependence than the clustering energies. This allows the calculation of more accurate heats of formation for larger clusters on the basis of calculated clustering energies.     

Theoretical characterization of structures and energies of benzene-(H2S)n and (H2S)n (n=1-4) clusters

An ab initio study was performed in clusters up to four H(2)S molecules and benzene using calculations at MP26-31+G(*) and MP2/aug-cc-pVDZ levels. Differences between both sets of calculations show the importance of using large basis sets to describe the intermolecular interactions in this system. The obtained binding energies reflect that benzene has not the same behavior in H(2)S as in water, pointing to a higher solubility of this molecule in H(2)S than in water. The Bz-cluster binding energy was fitted to an asymptotic representation with a maximum value of the energy of -8.00 kcal/mol that converges in a cluster with 12 H(2)S molecules. The obtained intermolecular distance in the Bz-H(2)S dimer is similar to the experimental value; however, the difference is much larger for the angles defining the orientation. The influence of benzene produces a distortion of the (H(2)S)(n) clusters, so the intermolecular distances change with regard to the (H(2)S)(n) isolated clusters. Frequency shifts are larger in clusters with benzene than without it. In the smallest clusters the shift associated to the stretching of the S-H bonded to benzene is the largest one, but for the cluster with three H(2)S molecules this stretching is combined with the other S-H stretching of the molecule so the resulting shift is not the largest one.     

Synthesis and characterization of novel monocarbollide exo-closo-(pi-arene)biruthenacarboranes [(PPh3)mClRu(eta6-C6H5R)Ru'CB10H11-n(OMe)n] (where R = H, m = 2, n = 1; R = mu-PPh2, m = 1, n = 0, 1)

The monocarbon carborane [Cs][nido-7-CB(10)H(13)] reacts with the 16-electron [RuCl(2)(PPh(3))(3)] in a solution of benzene/methanol in the presence of N,N,N',N'-tetramethylnaphthalene-1,8-diamine as the base to give a series of 12-vertex monocarbon arene-biruthenacarborane complexes of two types: [closo-2-[7,11-exo-RuClPPh(3)(mu,eta(6)-C(6)H(5)PPh(2))]-7,11-(mu-H)(2)-2,1-RuCB(10)H(8)R] (5, R = H; 6, R = 6-MeO; 7, R = 3-MeO) and [closo-2-(eta(6)-C(6)H(6))-10,11,12-[exo-RuCl(PPh(3))(2)]-10,11,12-(mu-H)(3)-2,1-RuCB(10)H(7)R(1)] (8a, R(1) = 6-MeO; 8b, R(1) = 3-MeO, inseparable mixture of isomers) along with trace amounts of 10-vertex mononuclear hypercloso/isocloso-type complexes [2,2-(PPh(3))(2)-2-H-3,9-(MeO)(2)-2,1-RuCB(8)H(7)] (9) and [2,5-(Ph(3)P)-2-Cl-2-H-3,9-(MeO)(2)-2,1-RuCB(8)H(6)] (10). Binuclear ruthenacarborane clusters of both series were characterized by a combination of analytical and multinuclear NMR spectroscopic data and by single-crystal X-ray diffraction studies of three selected complexes, 6-8. In solution, isomers 8a,b have been shown to undergo the isomerization process through the scrambling of the exo-[RuCl(PPh(3))(2)] fragment about two adjacent triangular cage boron faces B(7)B(11)B(12) and B(8)B(9)B(12).     

Density functional study of HO(H2O)n (n = 1–3) clusters

The hydrogen bonding complexes HO(H₂O)_n (n = 1–3) were completely investigated in the present study using DFT and MP2 methods at varied basis set levels from 6‐31++G(d,p) to 6‐311++G(2d,2p). For n = 1 two, for n = 2 two, and for n = 3 five reasonable geometries are considered. The optimized geometric parameters and interaction energies for various complexes at different levels are estimated. The infrared spectrum frequencies and IR intensities of the most stable structures are reported. Finally, thermochemistry studies are also carried out. The results indicate that the formation and the number of hydrogen bonding have played an important role in the structures and relative stabilities of different complexes. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Theoretical studies on structures of neptunyl carbonates: NpO2(CO3)m(H2O)n(q-) (m = 1-3, n = 0-3) in aqueous solution

Extensive ab inito computations have been carried out to study the equilibrium structure, infrared spectra, and bonding characteristics of a variety of hydrated NpO2(CO3)m(q-) complexes by considering the solvent as a polarizable dielectric continuum as well as the corresponding anhydrate complexes in the gas phase. The computed structural parameters and vibrational results at the MP2 level in aqueous solution are in good agreement with Clark et al.'s experiments and provide realistic pictures of the neptunyl complexes in an aqueous environment. Our computed hydration energies reveal that the complex with water molecules directly bound to it yields the best results. Our analysis of the nature of the bonding of neptunyl complexes provides insight into the nature of 6d and 5f bonding in actinide complexes.     

A density functional theoretical studies on the structures and aromaticities of （CH）n（BCO）6-n （n =0-6）

Density functional theoretical calculations have been made on the electronic structure of （CH）n（BCO）6-n（n = 0-6） at B3LYP/6- 311 ＋ G（d） level. The nuclear-independent chemical shifts （NICS） values calculated using the gauge-including atomic orbitals （GIAO） method were used to assess on the aromaticities of these molecules. The results shows that （CH）n（BCO）6n（n = 0-6） species are aromatic.     

Structures and properties of (H2GaN3)n (n = 1–4) clusters: A DFT study

Computations on the systems of (H₂GaN₃)_n (n = 1–4) are performed using the density functional theory (DFT)/B3LYP method with different basis sets. (H₂GaN₃)₂ possessing D_2h symmetry is found to exhibit the planar Ga₂N₂ ring structure. (H₂GaN₃)₃ involving a six‐membered Ga₃N₃ ring is found to exhibit two minima with very similar binding energies (ca. −235 ∼ −231 kJ · mol⁻¹). One minimum is the newly found boat‐like conformation possessing C_s symmetry. Another minimum possessing C_3v symmetry is the chair‐like conformation. (H₂GaN₃)₄ occurs in several structures with Ga₄N₄ eight‐membered ring structures that correspond to minima with slight energy differences among them. The structural changes of the clusters are large compared with the monomer. Frequency calculations are carried out on each optimized structure, and their infrared (IR) spectra are discussed. Thermodynamic properties demonstrate that the systems of H₂GaN₃ occur at dimer–trimer–tetramer equilibrium, and the trimer is the main component. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Syntheses and structures of the arylaluminum chalcogenides (ArAlE)2 (Ar = 2-(NEt2CH2)-6-MeC6H3, E = Se; Ar = 2,6-(NEt2CH2)2C6H3, E = Se, Te)

Two intramolecular stabilized arylaluminum dihydrides, (2-(NEt2CH2)-6-MeC6H3)AlH2 (1) and (2,6-(NEt2CH2)2C6H3)AlH2 (2), were prepared by reducing the corresponding dichlorides with an excess of LiAlH4 in diethyl ether. Reactions of 1 and 2 with elemental selenium afforded the dimeric arylaluminum selenides [(2-(NEt2CH2)-6-MeC6H3)AlSe]2 (3) and [(2,6-(NEt2CH2)2C6H3)AlSe]2 (4). Reaction of 2 with metallic tellurium gave the dimeric arylaluminum telluride [(2,6-(NEt2CH2)2C6H3)AlTe]2 (5). The possible reaction pathway is discussed, and molecular structures determined by single-crystal X-ray analyses are presented for 3 and 5.     

Threshold photoionization and density functional theory studies of the niobium carbide clusters Nb3C(n) (n = 1-4) and Nb4C(n) (n = 1-6)

We have used photoionization efficiency spectroscopy to determine ionization potentials (IP) of the niobium-carbide clusters, Nb3C(n) (n = 1-4) and Nb4C(n) (n = 1-6). The Nb3C2 and Nb4C4 clusters exhibit the lowest IPs for the two series, respectively. For clusters containing up to four carbon atoms, excellent agreement is found with relative IPs calculated using density functional theory. The lowest energy isomers are mostly consistent with the development of a 2 x 2 x 2 face-centered cubic structure of Nb4C4. However, for Nb3C4 a low-lying isomer containing a molecular C2 unit is assigned to the experimental IP rather than the depleted 2 x 2 x 2 nanocrystal isomer. For Nb4C5 and Nb4C6, interpretation is less straightforward, but results indicate isomers containing molecular C2 units are the lowest in energy, suggesting that carbon-carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. A double IP onset is observed for Nb4C3, which is attributed to ionization from the both the lowest energy singlet state and a meta-stable triplet state. This work further supports the notion that IPs can be used as a reliable validation for the geometries of metal-carbide clusters calculated by theory.     

Redox energetics of hypercloso boron hydrides B(n)H(n) (n = 6-13) and B12X12 (X = F, Cl, OH, and CH3)

The reduction potentials (E°(Red) versus SHE) of hypercloso boron hydrides B(n)H(n) (n = 6-13) and B(12)X(12) (X = F, Cl, OH, and CH(3)) in water have been computed using the Conductor-like Polarizable Continuum Model (CPCM) and the Solvation Model Density (SMD) method for solvation modeling. The B3LYP/aug-cc-pvtz and M06-2X/aug-cc-pvtz as well as G4 level of theory were applied to determine the free energies of the first and second electron attachment (ΔG(E.A.)) to boron clusters. The solvation free energies (ΔG(solv)) greatly depend on the choice of the cavity set (UAKS, Pauling, or SMD) while the dependence on the choice of exchange/correlation functional is modest. The SMD cavity set gives the largest ΔΔG(solv) for B(n)H(n)(0/-) and B(n)H(n)(-/2-) while the UAKS cavity set gives the smallest ΔΔG(solv) value. The E°(Red) of B(n)H(n)(-/2-) (n = 6-12) with the G4/M06-2X(Pauling) (energy/solvation(cavity)) combination agrees within 0.2 V of experimental values. The experimental oxidative stability (E(1/2)) of B(n)X(n)(2-) (X = F, Cl, OH, and CH(3)) is usually located between the values predicted using the B3LYP and M06-2X functionals. The disproportionation free energies (ΔG(dpro)) of 2B(n)H(n)(-) → B(n)H(n) + B(n)H(n)(2-) reveal that the stabilities of B(n)H(n)(-) (n = 6-13) to disproportionation decrease in the order B(8)H(8)(-) > B(9)H(9)(-) > B(11)H(11)(-) > B(10)H(10)(-). The spin densities in B(12)X(12)(-) (X = F, Cl, OH, and CH(3)) tend to delocalize on the boron atoms rather than on the exterior functional groups. The partitioning of ΔG(solv)(B(n)H(n)(2-)) over spheres allows a rationalization of the nonlinear correlation between ΔG(E.A.) and E°(Red) for B(6)H(6)(-/2-), B(11)H(11)(-/2-), and B(13)H(13)(-/2-).     

Structures, energetics, and vibrational spectra of H2O2...(H2O)n, n = 1-6 clusters: Ab initio quantum chemical investigations

Hydrogen-bonded heteroclusters of H(2)O(2)...(H(2)O)(n)(), with n varying from 1 through 6, have been investigated herein employing ab initio quantum chemical methods. For a given n, several energetically comparable conformers emerge as local minima on the potential energy surface (PES). All of the conformers obtained at restricted Hartree-Fock (RHF) and M?ller-Plesset second-order perturbation (MP2) levels of theory exhibit parallel trends in energy hierarchy. The effect of clustering by water on the modification in the vibrational frequencies has also been investigated and further, a many-body interaction-energy analysis is carried out providing insights into cooperativity in H(2)O(2)...(H(2)O)(n)() clusters.     

Theoretical studies of [MYR2]n isomers (M = B, Al, Ga; Y = N, P, As; R = H, CH3): structures and energetics of monomeric and dimeric compounds (n = 1, 2)

A series of group 13-15 compounds of the general formula [MYR(2)](n) (M = B, Al, Ga; Y = N, P, As; n = 1, 2; R = H, CH(3)) have been theoretically studied at the B3LYP/TZVP level of theory. The stability of different isomer structures is discussed to reveal the competitiveness of group 13-13, group 13-15, and group 15-15 bonding. Preferential bonding patterns and trends in the stability with respect to M and Y are also discussed. For the dimeric compounds, C(2v) symmetric [HMYH](2) rings are the lowest in energy, with the single exception of Ga(2)N(2)H(4), for which a somewhat unexpectedly C(2v) symmetric [GaNH(2)](2) ring is found to be the energy minimum, followed by the planar H(2)NGaGaNH(2) chain. The higher stability of the GaNH(2) bonding pattern in oligomer compounds may be rationalized in terms of the increasing stability of the oxidation state I as compared to that for the boron and aluminum analogues. Methylation significantly reduces the energetic differences between monomeric MYMe(2) MeMYMe, and Me(2)MY, isomers, especially for the AlP, AlAs, and GaAs systems, thus allowing a variety of structural types to be competitive in energy.     

Gas-phase ion mobilities and structures of benzene cluster cations (C6H6)n+, n = 2-6

Benzene clusters are generated by pulsed supersonic beam expansion, ionized by electron impact, mass-selected and then injected into a drift cell for ion mobility measurements in a helium buffer gas. The measured collision cross sections and theoretical calculations are used to determine the structures of the cluster cations (C(6)H(6))(n)(+) with n = 2-6. Density functional theory calculation, at an all-electron level and without any symmetry constraint, predicts that the dimer cation has two nearly degenerate ground state structures with the sandwich configuration more stable than the T-configuration by only 0.07 eV. The ion mobility experiment indicates that only one structure is observed for the mass-selected dimer cation at room temperature. The calculated cross section for the sandwich structure agrees very well (within 2.4%) with the experimental value. For the n = 3-6 clusters, the experiments suggest the presence of at least two structural isomers for each cluster. A Monte Carlo minimum-energy search technique using the 12-site OPLS potential for benzene is used to determine the structures of the lowest-energy isomers. The calculated cross sections for the two lowest-energy isomers of the n = 3-6 clusters agree well with the experimental results. The clusters' structures reveal two different growth patterns involving a sandwich dimer core or a pancake trimer stack core. The lowest-energy isomers of the n = 3-6 clusters incorporate the pancake trimer stack as the cluster's core. The trimer stack allows the charge to hop between two dimers, thus maximizing charge resonance interaction in the clusters. For larger clusters, the appearance of magic numbers at n = 14, 20, 24, 27, and 30 is consistent with the incorporation of a sandwich dimer cation within icosahedral, double icosahedral, and interpenetrating icosahedral structures. On the basis of the ion mobility results and the structural calculations, the parallel-stacked motif among charged aromatic-aromatic interactions is expected to play a major role in determining the structures of multi aromatic components. This conclusion may provide new insights for experimental and theoretical studies of molecular design and recognition involving aromatic systems.     

Microhydration of Cs+ ion: a density functional theory study on Cs+-(H2O)n clusters (n=1-10)

Structure, energy enthalpy, and IR frequency of hydrated cesium ion clusters, Cs+-(H2O)n (n=1-10), are reported based on all electron calculations. Calculations have been carried out with a hybrid density functional, namely, Becke's three-parameter nonlocal hybrid exchange-correlation functional B3LYP applying cc-PVDZ correlated basis function for H and O atoms and a split valence 3-21G basis function for Cs atom. Geometry optimizations for all the cesium ion-water clusters have been carried out with several possible initial guess structures following Newton-Raphson procedure leading to many conformers close in energy. The calculated values of binding enthalpy obtained from present density functional based all electron calculations are in good agreement with the available measured data. Binding enthalpy profile of the hydrated clusters shows a saturation behavior indicating geometrical shell closing in hydrated structure. Significant shifts of O-H stretching bands with respect to free water molecule in IR spectra of hydrated clusters are observed in all the hydrated clusters.     

叠氮化合物C6H6-n(N3)n(n=1~6)的密度泛函理论研究

运用密度泛函理论, 在B3LYP/6-31G*水平上, 对叠氮化合物C6H6-n(N3)n(n=1~6)进行理论计算, 并对所得到的几何结构进行了振动频率分析. 计算结果表明, 这些化合物是热力学稳定的. 基于自然键轨道理论, 分析了稳定结构的电荷分布及成键情况. 在不破坏苯环和叠氮基的原则下, 设计等键反应计算了这些化合物的生成热, 结果表明, 标题化合物的生成热都很高, 且随着叠氮基数目的增加而线性增大.     

A density functional study on nitrogen-doped carbon clusters CnN3- (n=1-8)

Using molecular graphics software, we designed numerous models of CnN3- (n=1-8). Geometry optimization and calculation on vibration frequency were carried out by the B3LYP density functional method. After comparison of structure stability, we found that the structures of ground-state CN3- and C2N3- are bent chains with a nitrogen atom at either end, whereas when n=3-8, the ground-state clusters show three branches, each with a nitrogen atom located at the end. When n=5-8, the longest branch of CnN3- is polyacetylenelike. When n=5 or 7, the longest branch is connected to the central sp2 carbon in a nonlinear manner. The CnN3- (n=1-8) with an even number of carbon atoms are more stable than those with odd numbers, matching the peak pattern observed in laser-induced mass spectra of CnN3-. The trend of such odd/even alternation is explained based on concepts of bonding characteristics, electron affinities, and incremental binding energies.     

The structures and electronic states of zinc-water clusters Zn(n)(H2O)(m) (n = 1-32 and m = 1-3)

Ab initio and Density Functional Theory (DFT) calculations have been carried out for zinc-water clusters Zn(n)-(H2O)(m) (n = 1-32 and m = 1-3, where n and m are the numbers of zinc atoms and water molecules, respectively) to elucidate the structure and electronic states of the clusters and the interaction of zinc cluster with water molecules. The binding energies of H2O to zinc clusters were small at n = 2-3 (2.3-4.2 kcal mol(-1)), whereas the energy increased significantly in n = 4 (9.0 kcal mol(-1)). Also, the binding nature of H2O was changed at n = 4. The cluster size dependency of the binding energy of H2O accorded well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in surface regions of the zinc cluster have a positive charge, whereas those in the interior region have a negative charge with the large electron population in the 4p orbital. The interaction of H2O with the zinc clusters were discussed on the basis of the theoretical results.     

PdS2C2(COOMe)2](6n) (n = 0, -1, -2, -3, -4): hexanuclear homoleptic palladium dithiolene complexes

Reaction of a stoichiometric equivalent of the zinc-dithiolene complex, (tmeda)ZnS2C2(COOMe)2 (tmeda = tetramethylethylenediamine), with (MeCN)2PdCl2 results in a 1:1 homoleptic dithiolene that forms the hexanuclear cluster [PdS2C2(COOMe)2]6 (1). X-ray structure analysis of 1 indicates a Pd6S12 core comprised of six face-centered palladium atoms and 12 edge-centered sulfur atoms situated on an imaginary approximate cube. Complex 1 undergoes four distinct and reversible one-electron redox steps in dichloromethane at -186, -484, -1174, and -1524 mV versus a standard calomel electrode (ferrocenium+/ferrocene redox couple 409 mV). The two-electron reduction product of 1, [Bu4N]2[(PdS2C2(COOMe)2)6] (2), has been chemically isolated and characterized.     

Infrared spectra of the Li+-(H2)n (n=1-3) cation complexes

The Li+-(H2)n n=1-3 complexes are investigated through infrared spectra recorded in the H-H stretch region (3980-4120 cm-1) and through ab initio calculations at the MP2/aug-cc-pVQZ level. The rotationally resolved H-H stretch band of Li+-H2 is centered at 4053.4 cm-1 [a -108 cm-1 shift from the Q1(0) transition of H2]. The spectrum exhibits rotational substructure consistent with the complex possessing a T-shaped equilibrium geometry, with the Li+ ion attached to a slightly perturbed H2 molecule. Around 100 rovibrational transitions belonging to parallel Ka=0-0, 1-1, 2-2, and 3-3 subbands are observed. The Ka=0-0 and 1-1 transitions are fitted by a Watson A-reduced Hamiltonian yielding effective molecular parameters. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.056 A increasing by 0.004 A when the H2 subunit is vibrationally excited. The spectroscopic data are compared to results from rovibrational calculations using recent three dimensional Li+-H2 potential energy surfaces [Martinazzo et al., J. Chem. Phys. 119, 11241 (2003); Kraemer and Spirko, Chem. Phys. 330, 190 (2006)]. The H-H stretch band of Li+-(H2)2, which is centered at 4055.5 cm-1 also exhibits resolved rovibrational structure. The spectroscopic data along with ab initio calculations support a H2-Li+-H2 geometry, in which the two H2 molecules are disposed on opposite sides of the central Li+ ion. The two equivalent Li+...H2 bonds have approximately the same length as the intermolecular bond in Li+-H2. The Li+-(H2)3 cluster is predicted to possess a trigonal structure in which a central Li+ ion is surrounded by three equivalent H2 molecules. Its infrared spectrum features a broad unresolved band centered at 4060 cm-1.